1. Field of the Invention
The present invention relates to a leak diagnosis system and a leak diagnosis method.
2. Description of Related Art
In a known purge apparatus, fuel gas, which is generated in a fuel tank of a vehicle is adsorbed by a canister and is then purged into an air intake passage of an internal combustion engine of the vehicle through use of a negative pressure in the intake air passage upon opening of a purge valve installed in a purge passage, which communicates between the canister and the air intake passage. Therefore, leakage of the fuel gas into the surrounding atmosphere is effectively limited. A leak diagnosis system has been proposed to limit a leaked state of the purge apparatus where the purge apparatus is left to leak the fuel gas into the atmosphere. Specifically, the leak diagnosis system diagnoses whether a leak hole exists in the purge apparatus based on an internal pressure of the purge apparatus at the time of depressurizing or pressurizing the interior of the purge apparatus.
Japanese Unexamined Patent Publication No. 2004-293438 discloses such a leak diagnosis system. In the leak diagnosis system of Japanese Unexamined Patent Publication No. 2004-293438, a vacuum pump, which is communicated with the interior of the purge apparatus, is provided, and a suction capacity of the vacuum pump is set to maintain the internal pressure of the purge apparatus to a fixed value when a leak hole, which has a predetermined inner diameter (e.g., a diameter of 0.5 mm), exists in the purge apparatus.
During the diagnosis, the purge apparatus is sealed from the surrounding atmosphere to form a sealed system, and the vacuum pump is driven for a predetermined time period. At this time, when the internal pressure of the purge apparatus becomes the fixed value, it is determined that a leak hole, which has an inner diameter equal to or greater than the predetermined inner diameter, exists in the purge apparatus. Alternatively, when the internal pressure of the purge apparatus becomes higher than the fixed value, it is determined that a leak hole, which has an inner diameter greater than the predetermined inner diameter, exists in the purge apparatus. Also, when the internal pressure of the purge apparatus becomes lower than the fixed value before elapse of the predetermined time period, it is determined that a leak hole does not exist in the purge apparatus.
When the suction capacity of the pump is set to a fixed capacity, like in the case of Japanese Unexamined Patent Publication No. 2004-293438, it is possible to determine whether the leak hole, which has the inner diameter equal to or greater than the predetermined inner diameter, exists in the purge apparatus. However, in a case where a remaining fuel quantity in the fuel tank is relatively small, when the suction capacity of the pump is set to the fixed capacity, it takes a relatively long time period to reduce the internal pressure of the purge apparatus in comparison to a case where the remaining fuel quantity in the fuel tank is relative large, so that the diagnosis takes a relatively long time period. Furthermore, the above diagnosis needs to be performed in a stable state of the vehicle where a pressure change is not induced by swing movement of fuel inside the fuel tank. Thus, the above diagnosis is performed during a stop period or an idling period of the engine. Therefore, when the diagnosis takes a long time period, a driver may possible start the engine or may possibly start driving of the vehicle before completion of the diagnosis to cause the unstable state of the vehicle. As a result, the diagnosis may be interrupted.
In order to quickly reduce the internal pressure of the purge apparatus even in the case of the relatively small remaining fuel quantity in the fuel tank, it is conceivable to use a vacuum pump of a higher capacity. However, when the vacuum pump of the higher capacity is simply used, the internal pressure of the purge apparatus is kept reduced even when the leak hole, which has the inner diameter equal to or greater than the predetermined inner diameter, exists in the purge apparatus. Thus, the leak hole in the purge apparatus may not be detected in such a case.
In the above leak diagnosis, presence of the leak hole in the purge apparatus is diagnosed by measuring the pressure inside the purge apparatus. The pressure inside the fuel tank may change due to a swing movement of fuel in the fuel tank or a change in the atmospheric pressure at the time of traveling along a climbing road. A change in such an external state causes a change in the pressure inside the purge apparatus. When this happens, accurate diagnosis of the leak hole cannot be made. That is, in the leak diagnosis, the pump should be operated to change the pressure inside the purge apparatus only when the pressure inside the purge apparatus is stabilized.
Thus, in Japanese Unexamined Patent Publication No. 2004-293438, the leak diagnosis is executed in an idling state or a stop state of the engine. However, right after stopping of the engine, the pressure inside the fuel tank is not stabilized. Thus, the leak diagnosis in the stop state of the engine should be executed after stabilization of the pressure inside the fuel tank. In Japanese Unexamined Patent Publication No. 2004-293438, the time period, which is required to stabilize the pressure inside the fuel tank, is set to be a predetermined time period (3 to 5 hours), and the leak diagnosis is executed after the elapse of this predetermined time period since the time of stopping the engine.
As described above, Japanese Unexamined Patent Publication No. 2004-293438 teaches that the pressure inside the purge apparatus is stabilized after the elapse of the predetermined time period since the time of stopping the engine. However, in reality, the time period required to stabilize the pressure inside the purge apparatus varies depending on a remaining fuel quantity in the fuel tank, the surrounding temperature and/or the presence of the leak hole. Thus, the above predetermined time period needs to include a surplus time, which is required to sufficiently stabilize the pressure, in view of aging of the system even if the predetermined time period is determined based on the worst environmental conditions, which tend to increase the time period required to stabilize the pressure inside the purge apparatus. Therefore, in some cases, it could happen that the leak diagnosis cannot be executed although the pressure inside the purge apparatus has been already stabilized a long time ago.
Furthermore, in the case where the leak diagnosis cannot be executed until 3 to 5 hours elapse since the time of stopping the engine, when the engine is restarted, the leak diagnosis cannot be made, and the required number of times of leak diagnosis cannot be achieved.